1. Field of Invention
The present invention relates to a control circuit of a switching regulator wherein a confirmation signal related to an upper gate switch is generated to confirm that the upper gate switch has been turned off, to safely operate a lower gate switch, and to avoid shoot-through. The present invention also relates to a control method for controlling a switching regulator, and a transistor device for use in a control circuit of a switching regulator.
2. Description of Related Art
FIG. 1 shows a schematic diagram of a conventional switching regulator control circuit, wherein an upper gate switch UG and a lower gate switch LG of the switching regulator are driven by an upper gate driver circuit 13 and a lower gate driver circuit 14, as shown in the figure. The upper gate driver circuit 13 includes a latch circuit 131 and a driver circuit 132, and the lower gate driver circuit 14 includes a latch circuit 141 and a driver circuit 142. The upper gate driver circuit 13 is controlled by a level shift circuit 12, and the lower gate driver circuit 14 is controlled by a control logic circuit 11. When an input voltage Vin is a relatively high voltage, for example 400 volts, the level shift circuit 12 is required for providing a correct operating voltage to the upper gate driver circuit 13 in order to properly drive the upper gate switch UG. Because the input voltage Vin is high, a high voltage shielding device such as a lateral diffused metal-oxide-semiconductor (LDMOS) device must be used as the first transistor Q1 and the second transistor Q2 in the level shift circuit 12. The transistors Q1 and Q2 occupy a relatively large area of the whole circuit. One example of this kind of switching regulator is disclosed in U.S. Pat. No. 5,446,300.
The circuit shown in FIG. 1 has a risk of shoot-through. The so-called shoot-through means that the upper gate switch UG and the lower gate switch LG are both ON at the same time, causing the input voltage Vin to short to ground. The reason why shoot-through happens is thus. When the control logic circuit 11 generates a control signal (i.e., the resetting signal RESET) to turn OFF the upper gate switch UG, due to operation delay of the level shift circuit 12 and the upper gate driver circuit 13, the actual timing when the upper gate switch UG is turned OFF is later than the level switching timing of the resetting signal RESET. Therefore, if the control logic circuit 11 turns ON the lower gate switch LG at the same time as it outputs the resetting signal RESET, shoot-through will occur. In this regard, the aforementioned U.S. Pat. No. 5,446,300 does not mention any mechanism to protect the circuit from shoot-through. Presently, one typical solution is to delay a fixed dead time after the control logic circuit 11 outputs the resetting signal RESET, for example, to turn ON the lower gate switch LG after 500˜1000 nanoseconds (ns) from when the control logic circuit 11 outputs the resetting signal RESET, to prevent from shoot-through. This open loop solution is not satisfactory because the dead time is set manually and usually not optimized. If the dead time is set too long, the efficiency is not optimized; if the dead time is set too short, the risk of shoot-through remains.
In view of the above drawbacks, the present invention provides a switching regulator driver circuit which samples a signal related to the upper gate switch UG to confirm that the upper gate switch UG is OFF, so that the lower gate switch LG can be turned ON safely. Thus, in avoiding shoot-through, the circuit operates with optimized efficiency, and the device or circuit area is not significantly increased as compared to the prior art. The present invention also provides a control method of the switching regulator and a transistor device for use in a control circuit of a switching regulator.